1. Field
The present disclosure relates generally to a power conversion system and, more specifically, to a current limiter circuit that can be used in a power conversion system having a triac dimmer.
2. Discussion of the Related Art
Many electronic devices, such as cell phones, laptop computers, etc., use direct current (dc) power to operate. Conventional wall outlets generally deliver a high-voltage alternating current (ac) power that needs to be transformed to dc power in order to be used as a power source by most electronic devices. In a switch mode power converter, also referred to as a switch mode power supply, a switch is used to provide a desired dc power by either switching at a variable duty ratio (typically the ratio of on-time of the switch to total switching period) or by varying the number of ON and OFF cycles per unit time. A switch mode power converter may also employ a controller to provide output regulation by sensing the output and controlling the switch of the power converter in a closed loop. Switch mode power converters are commonly used due to their high efficiency, small size, and the safety and protection features that are provided. One important safety consideration for a switch mode power converter is the protection against power line surges that may damage the power converter and/or the sensitive load(s) coupled to the output of the power converter. This type of protection may be referred to as surge protection.
Switch mode power converters have recently found wide use in applications involving high-efficiency light sources. Example applications include electronic ballasts for fluorescent or other discharge lamps, light-emitting diode (LED) drivers (e.g., power conversion systems that provide regulated power to LED lamps), etc. LEDs have become popular due to their longer life and their recent performance improvement in light spectrum (e.g., color), Volt-Ampere characteristics, and Lumens per Watt delivery. As such, safe and efficient LED drivers have recently received considerable attention in the lighting industry. LED drivers that include switch mode power converters, which may also be referred to as off-line LED drivers, may be exposed to power line surges due to lightning or other interruptions. Therefore, it may be desired for an off-line LED driver to provide a surge protection feature.
Some lighting applications include a feature that allows the user to control the brightness of a lighting device. Specifically, in lighting applications, reducing the brightness of the lighting device may be referred to as dimming and may be accomplished by limiting the power supplied to the lighting device. In one example, dimming can be realized by implementing phase dimming, where a portion of the ac input voltage, also referred to as an ac line signal, is blocked from being received by the lighting device. This reduces the amount of power delivered to the lighting device, thereby reducing the brightness of the device. In phase dimming, the portion of the input voltage that is blocked can be measured by a phase angle that represents a portion of a period of the input voltage measured in degrees. One period may be defined as one complete line cycle and may correspond to 360 degrees. Similarly, half of the period of the input voltage may be referred to as a half line cycle and may correspond to 180 degrees.
One device that may be used to implement phase dimming in a lighting system is a triac dimmer. Specifically, a triac may include a semiconductor component that behaves as a switch used to block a portion of the input voltage from the input of the power converter. In operation, when the triac is disengaged (turned off), the ac line signal is blocked from the input of the power converter and the current to the power converter is substantially restricted. Conversely, when the triac is engaged (turned on), the ac line signal is unblocked from the input of the power converter and the current is permitted to conduct to the power converter. The triac may remain engaged and continue to conduct current until the current in the triac drops below a threshold value commonly known as a holding current.
Implementing phase dimming in an off-line LED driver with a triac dimmer may prove challenging for several reasons, with the triac itself being one of them. For example, when the triac fires (e.g., turns on) during each half cycle of the input voltage, the voltage across the switch suddenly increases (typically, in the form of a spike) leading to ringing in the triac current, which may ultimately cause the triac to misfire. Specifically, because of the ringing, the triac may conduct insufficient current (less than the holding current) to remain engaged and may prematurely turn off. Triac misfires usually happen at different times in consecutive input voltage cycles and, as such, energy delivered to the LED lamp varies from one cycle to another. This may result in a perceivable fluctuation of light output by the LED lamp (e.g., flickering in the LED lamp). As a remedy, a resistor may be used in series with the triac dimmer as a damper to dampen the ringing in the triac current. However, use of the resistor alone may lower the overall efficiency of the off-line LED driver.